1. Field of the Invention
This invention is concerned with an improved catalytic process for the demetalation and desulfurization of petroleum oils, preferably those residual fractions with undesirably high metals and/or sulfur and/or Conradson Carbon Residue (CCR) contents. More particularly, this invention relates to a multi-stage hydrotreatment process for reducing high metals, sulfur and CCR contents of petroleum oils, again preferably those containing residual hydrocarbon components, by the use of a sequential combination of catalytic compositions which is especially effective for such a purpose.
2. Description of the Prior Art Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals, sulfur and CCR content. This comes about because practically all of the metals and CCR present in the original crude remain in the residual fraction, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals and CCR content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalyst processing such as catalytic cracking and hydrocracking. The metal contaminants deposit on the special catalysts for these cracking processes and cause the premature aging of the catalyst and/or formation of inordinate amounts of coke, dry gas and hydrogen. CCR, a measure of a molecule's tendency to coke rather than crack and/or distill, is also an undesirable property for charge streams processed by catalytic cracking. Under the high temperature employed in catalytic cracking, molecules high in CCR thermally and/or catalytically degrade to coke, light gases, and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolytic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leaves behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of one to ten atmospheres. The economic value of the coke by-product is determined by its quality, especially its fulfur and metals content. Excessively high levels of these contaminants make the coke useful only as low valued fuel. In contrast, cokes of low metals content, for example up to about 100 ppm (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high valued metallurgical, electrical and mechanical applications.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content is many cases is unacceptable for environmental reasons.
At present, catalytic cracking is generally done utilizing hydrocarbon charge stocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking charge stocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since these cracking charge stocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree. to 1500.degree. F., a pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 1000 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a charge stock's "metals factor". This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
Conventionally, a charge stock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25, or even 2.5 to 50, may be used to blend with or as all of the feedstock to a catalytic cracker, since charge stocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance, with the new fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions (having a metals factor of about 2.5 to 50) suitable for cracking charge stocks.
Metals and sulfur contaminants would present similar problems with regard to hydrocracking operations which are typically carried out on charge stocks even lighter than those charged to a cracking unit. Typical hydrocracking reactor conditions consist of a temperature of 400.degree. to 1000.degree. F. and a pressure of 100 to 3500 psig.
It is evident that there is considerable need for an efficient method to reduce the metals and/or sulfur and/or CCR content of hydrocarbons, and particularly of residual petroleum fractions. While the technology to accomplish this for distillate fractions has been advanced considerably, attempts to apply this technology to residual fractions generally fail due to very rapid deactivation of the catalyst, presumably by metals contaminants and coke deposition.
U.S. Pat No. 3,696,027 suggests sequentially contacting the feedstream with three fixed beds of catalysts having decreasing macroporosity along the normal direction of feed flow. "Macroporosity" denotes catalyst pores greater than about 500 Angstroms (A) in diameter. It is said to be strongly related to the capacity of catalyst particles to retain metals removed from a heavy hydrocarbon stream contaminated with organometallic compounds. The catalyst particles of the first bed of the '027 process have at least 30 vol. % macropores; the catalyst particles of the second bed have between 5 and 40 vol. % macropores; and the catalyst particles of the third bed have less than 5 vol. % macropores. The patent also teaches that the three fixed beds have progressively more active desulfurization catalysts along the normal direction of flow. The third catalyst bed (which contains the most active desulfurization catalyst) contains high surface area particles having an average pore diameter of at least 50 A, preferably at least 80 A, and more preferably at least 100 A, in order to lengthen the desulfurization run.
U.S. Pat. No. 3,730,879 discloses a two-bed catalytic process for the hydrodesulfurization of crude oil or a reduced fraction, in which at least 50% of the total pore volume of the first bed catalyst consists of pores in 100-200 A diameter range and in which less than 45% of the total pore volume of the second bed catalyst consists of pores in the 100-200 A diameter range. According to the '879 process, demetalation activity increases and desulfurization activity decreases along the normal direction of flow. The patent further suggests a two-catalyst-bed system with increasing average pore diameters and decreasing surface areas.
U.S. Pat. No. 3,766,058 also teaches a two-stage process for hydroprocessing a heavy hydrocarbon feedstock in which the second stage catalyst has a larger pore diameter than the first stage catalyst. Similar teachings are found in U.S. Pat. No. 3,830,720 and U.S. Pat. No. 4,048,060.
U.S. Pat. No. 3,876,530 discloses a multi-stage catalytic process for desulfurizing residual oils in which the initial stage catalyst has a relatively low proportion of hydrogenation metals and in which the final stage catalyst has a relatively high proportion of hydrogenation metals.
U.S. Pat. No. 3,931,052 suggests a two-stage process wherein the first stage catalyst has a strong selectivity for sulfur removal and the second stage catalyst has a strong selectivity for metals removal (U.S. Pat. No. 3,931,052 at col. 4, lines 32-43). The active desulfurization catalyst has at least 50% of its pore volume in the 30 to 100 A diameter range. The active demetalation catalyst has pores substantially distributed over a narrow 180 to 300 A diameter range (not less than 65% of the total pore volume is contained in pores having a diameter between 180 to 300 A).
U.S. Pat. No. 3,977,962 discloses a two-stage hydro-conversion process using catalysts having certain pore sizes, surface areas and pore volumes. Both stages employ high surface area catalysts (200-600 m.sup.2 /g). The second stage catalyst generally has a smaller average pore diameter and surface area relative to the first stage catalyst.
U.S. Pat. No. 4,016,067 discloses a process for demetalation and desulfurization of petroleum oils in two stages with sequentially decreasing average pore diameters and increasing surface areas. The first catalyst has at least about 60% of its pore volume in 100-200 A pores, at least about 5% of its pore volume in pores greater than 500 A, and a surface area of up to about 110 m.sup.2 /g. The second catalyst has at least 50% of its pore volume in 30 to 100 A pores and a surface area of at least 150 m.sup.2 /g.
U.S. Pat. No. 4,054,508 discloses a three-stage process for demetalation and desulfurization of petroleum oils wherein the first and second stages contain catalysts as described in related U.S. Pat. No. 4,016,067 (supra) and the third stage comprises a second, smaller bed of the first stage catalyst.
An objection of this invention is an improved process for the removal of metal and/or sulfur and/or CCR contaminants from hydrocarbons especially heavy hydrocarbons such as residual petroleum fractions. Another object of this invention is an improved method for desulfurizing and demetalizing hydrocarbon fractions having significant metal and/or sulfur contaminant contents to produce suitable chargestocks for cracking, hydrocracking, or coking units or to produce a suitable fuel oil or fuel oil blend stock.